The present invention relates to a method of making fiber optic couplers and is especially applicable to the fabrication of 1.times.N couplers which couple light from one optical fiber to N optical fibers, wherein N is a number equal to 2 or more. In addition to performing the coupling function, such devices can also be designed to perform such functions as wavelength division, multiplexing, filtering and the like. In couplers wherein N is greater than two, a usual requirement is that the power be equally coupled from the input fiber to the N output fibers.
Methods of making multimode 1.times.N couplers are disclosed in U.S. Pat. No. 4,083,625 (Hudson) and in GB No. 2,023,874 A. Both of these documents teach methods that initially require fusing together and stretching the end portions of first and second optical fibers. In accordance with the Hudson patent, the common endface of the fused first and second fibers is fused to the endface of a third optical fiber. The British document teaches that a spheroidal lens is formed on the common end of the tapered first and second fibers, and a third fiber is positioned such that light radiating therefrom is coupled by the lens to the first and second fibers. These couplers are not suitable for single-mode fibers.
Single-mode couplers have been made by forming N.times.N couplers and severing and/or terminating N-1 fibers at one end of the device. For example, an N.times.N single-mode optical fiber coupler can be formed in accordance with the teachings of European patent application Ser. No. 0302745, published 08 Feb. 89. A plurality of suitably prepared glass fibers, each having a core and cladding, are disposed within the longitudinal aperture of a glass capillary tube. Each fiber extends beyond at least one end of the tube to form a connection pigtail. The pigtails are provided with the same type and diameter of plastic coating as the optical fibers of the system in which the device is intended to be used so that the device can be easily integrated into the system. The midregion of the tube is heated and collapsed about the fibers, and the central portion of the collapsed midregion is stretched to reduce the diameter thereof. To form a 1.times.2 coupler, for example, two fibers are employed to initially form a 2.times.2 coupler, and one of the coupler pigtails or legs is terminated. In this type of coupler, the terminated leg is located external to the glass tube.
A requirement for certain coupler applications is that the terminated leg be such that the back reflection of optical power therefrom is substantially reduced. A common specification for such applications is that the reflected power be no more than -50 dB.
The preparation of the optical fibers for the above-described method has involved removing the coating from that portion of the fiber which is positioned at the midregion of the tube. The coated portions of the fibers are pulled through the tube aperture in order to position the uncoated region in the center of the tube prior to the tube collapse step. The coated portions of each fiber extend into the ends of the tube aperture to hold the fibers in proper alignment while the tube is collapsed thereon. The aperture must therefore be large enough to accept the coated portions of the fibers. When the aperture diameter is relatively large, whereby the tube must undergo an inordinate amount of collapse prior to the time that it engages the fibers therein, bubbles are more likely to form in the coupling region, and/or glass is more likely to flow between the fibers during the tube collapse step. These effects adversely affect optical characteristics such as attenuation and coupling ratio.
When N is large, it becomes more difficult to make 1.times.N couplers by severing and/or terminating pigtails extending from an N.times.N coupler. In such a coupler the input fiber must be centrally located with respect to the output fibers in order to obtain the best distribution of optical power to all output ports. Each of the N input ports of the initially formed N.times.N coupler would have to be measured in order to determine the centrally located port. A 16.times.16 coupler would require 256 measurements.